1. Field of the Invention
The present invention relates to a frame and, more particularly, to a bicycle rear frame.
2. Description of the Related Art
A conventional bicycle rear frame in accordance with the prior art shown in FIGS. 5-7 comprises a mandrel 21 having two opposite threaded ends 210, a hub 20 rotatably mounted on the mandrel 21, a freewheel 22 secured on one side of the hub 20 to rotate the hub 20, two rear fork end members 11 each having a rear end mounted on a respective threaded end 210 of the mandrel 21 and each rested on the hub 20, two rear stays 10 each having a rear portion mounted on a front end of a respective rear fork end member 11, two washers 23 each mounted on a respective threaded end 210 of the mandrel 21 and each rested on a respective rear fork end member 11, and two locking nuts 24 each screwed onto a respective threaded end 210 of the mandrel 21 and each rested on a respective washer 23. Each of the rear fork end members 11 has a side formed with a mounting opening 12 mounted on the respective threaded end 210 of the mandrel 21.
In assembly, a rear wheel is mounted on the hub 20, and the freewheel 22 meshes with a chain 72 which meshes with a chainwheel. Then, the mandrel 21 is mounted between the rear fork end members 11, and each of the threaded ends 210 of the mandrel 21 is inserted into the mounting opening 12 of the respective rear fork end member 11. At this time, each of the threaded ends 210 of the mandrel 21 is movable in the mounting opening 12 of the respective rear fork end member 11 to adjust the tension of the chain 72. Then, each of the washers 23 is mounted on the respective threaded end 210 of the mandrel 21 and rested on the respective rear fork end member 11. Then, each of the locking nuts 24 is screwed onto the respective threaded end 210 of the mandrel 21 and rested on the respective washer 23 to press the respective washer 23 and the respective rear fork end member 11 toward the hub 20, so that the rear fork end members 11 and the hub 20 are combined together and the hub 20 is clamped between the rear stays 10.
In operation, when the chainwheel is rotated by a crank which is rotated by pedals, the chain 72 is driven by the chainwheel to rotate the freewheel 22 which rotates the hub 20 to rotate the wheel so as to move the bicycle forward.
However, when the freewheel 22 and the hub 20 are pulled forward by the chain 72 during forward movement of the bicycle, the mandrel 21 is driven by the hub 20 to move forward to displace the freewheel 22 and the hub 20 toward the rear stays 10, so that the chain 72 easily becomes loosened from the freewheel 22. If the locking nuts 24 are tightened to press the rear fork end members 11 closely to lock the freewheel 22 and the hub 20 exactly to prevent the chain 72 from becoming loosened, each of the rear fork end members 11 made of an aluminum alloy is easily deformed or worn out due to an excessive force applied by each of the locking nuts 24 on the respective rear fork end member 11, thereby decreasing the lifetime of the rear fork end members 11.